The present invention relates to a method for applying a fluid to a substrate, notably to a method for applying ink or an adhesive to a paper or plastics sheet, carton or the like; and to apparatus for use in that method.
Conventionally, coatings of ink or adhesives are applied to substrates by means of a roller applicator. However, the applicator applies a given set pattern or swath of material and it is necessary to change the roller if a different application pattern is required. Also, the ink or adhesive tends to dry out on the surface of the roller when application is interrupted, giving rise to uneven application and/or blockage problems when the application process is re-started.
It has been proposed to apply fluids by means of a spray nozzle. This method still suffers from problems due to drying out where the application is repeatedly interrupted. Furthermore, problems are also encountered with accurate placement of the spray onto the substrate due to drift of the spray droplets in the air stream used to form the spray.
In order to overcome this problem, it has been proposed to give the fluid being sprayed and the substrate opposite electrical charges so that the droplets from the nozzle are guided by electrostatic forces onto the substrate. However, this process is primarily applied to substrates which are electrically conductive. Where a non-conductive substrate is used, for example paper or plastics articles, it is usually necessary to provide a second electrode behind or adjacent the substrate to provide a co-operating target charge to guide the fluid droplets in the desired direction. Furthermore, such methods can not be applied to the formation of accurately defined patterns on the substrate since the fluid is formed into droplets of widely varying sizes and velocities and there is little or no directional control of specific droplets in the spray. Thus, such methods cause localised over or under application of the fluid and mist formation leading to loss of the material from the desired spray path.
It has been proposed in U.S. Pat. No. 3,416,153 to apply inks to a substitute by a method in which the ink is fed under pressure to a nozzle to form a jet of ink issuing from the nozzle. Due to surface tension effects, this jet breaks up into individual droplets which are then applied to the substrate by allowing them to pass through a hole in a masking plate between the nozzle and the substrate. The droplet formation can be assisted by applying vibration and/or pressure pulses to the jet, eg. using a piezoelectric crystal. When a printed image is not required, a charge is applied to the droplets by passing them past a charge electrode operated at a voltage of up to 1000 volts with respect to the fluid issuing from the nozzle. This causes the droplets to repel each other and thus form a divergent spray of the ink which is no longer directed at the hole in the mask. Hence little or none of the ink passes through the mask to strike the substrate. The form of the image printed on the substrate is controlled by selecting which droplets are allowed to reach the substrate and by movement of the substrate to select the position at which the droplet strikes the substrate.
In an alternative form of such an on/off method for controlling the feed of droplets to the substrate described in U.S. Pat. Nos. 3,673,601 and 3,916,421, the mask is omitted and the divergent spray of droplets is directed to a catcher device by applying an electric field to the spray using a second electrode operated separately from and with a polarity opposed to that of the first electrode. The catcher can be a trough or the like into which the spray is directed. However, due to problems of build up of ink on the second electrode, it was considered necessary to form one of the electrodes as a porous material and to suck the ink attracted onto that electrode through it for collection. The collected ink was discarded and could not be re-used, due to contamination of the ink and solids therein.
Such a method suffers from the need for complex systems to control the relative movement of the substrate with respect to the nozzle so as to position the droplet in the desired position on the substrate. In practice, such methods have only found use where linear images are to be formed on the substrate, eg. for use in plotters, and have not proved practicable for other uses. Furthermore, such methods have been limited to the use of small nozzle orifices, typically less than 25 microns in diameter. This is due to the fact that the flight paths of the droplets are not accurately controlled and any errors become visually obtrusive with larger sized droplets.